Re: Clarification to pg_upgrade docs on reverting to old cluster

Reading the pg_upgrade reference page, I get the feeling that one of the
bullets under "Reverting to old cluster" is a bit thin on detail to be helpful
to newcomers:

    "If you ran pg_upgrade with --link, the data files are shared between the
    old and new cluster.  If you started the new cluster, the new server has
    written to those shared files and it is unsafe to use the old cluster."

This is perfectly correct, but it fails to provide information on what to do
next in case reverting is in fact what the user wants.  The attached patch adds
a short sentence saying the old cluster should be restored from backups at this
point.